JSFiddle - React, Tailwind, and code Playground

JavaScript

function getSizes(baseSize, sVariance) {
    var sizes = [];
    for (i = 0; i < 5; i++) {
        sizes[i] = Math.ceil(baseSize - (Math.random() * sVariance));
    }
    return sizes;
}

function calculateOpacityValues(baseSize, sVariance) {
    var min = 0.25, max = 1,
        step = 0.75 / sVariance, i,
        opacityValues = {};
    for (i = 0; i <= sVariance; i++) {
        opacityValues[baseSize - i] = min + step * i;
    }
    return opacityValues;
}

function getOpacities(sizes, opacityValues) {
    // create a map of opacities
    // and your expected values
    var opacities = [];
    sizes.forEach(function(value, index) {
        opacities[index] = opacityValues[value];
    });
    return opacities;
}

var baseSize = 60, sVariance = 3, 
    sizes = getSizes(baseSize, sVariance);
console.log(sizes);
// use the map to get the opacity
console.log(getOpacities(sizes, calculateOpacityValues(baseSize, sVariance)));